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A calorie is a unit of measurement for energy. In most fields, it has been replaced by the joule, the SI unit of energy. However, the kilocalorie or calorie remains in common use for the amount of food energy. The calorie was first defined by Professor Nicolas Clément in 1824 as a kilogram-calorie and this definition entered French and English dictionaries between 1842 and 1867.

The calorie was never an SI unit. Modern definitions for calorie fall into two classes:

The small calorie or gram calorie approximates the energy needed to increase the temperature of 1 gram of water by 1 °C. This is about 4.184 joules.

The large calorie or kilogram calorie approximates the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.184 kJ, and exactly 1000 small calories.

In some scientific contexts such as physics and chemistry, the name "calorie" refers strictly to the gram calorie, and this unit has the symbol cal (a symbol also used by many for the large calorie). Prefixes are used with this name and symbol, so that the kilogram calorie is often known as the "kilocalorie" and has the symbol kcal.

In the medical sciences and non-scientific contexts the calorie is equal to a kilocalorie in the physics or chemistry sense, and is occasionally referred to as a Calorie (capital "C") in an attempt to distinguish it. This has been somewhat ineffective, partly because the convention is not used outside this context, and partly because it results in ambiguity when the word appears at the beginning of a list or sentence. Thus it has to be inferred from the context that the small calorie is not intended.

The conversion factor among calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units). See "Versions" below for explanation of units.

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The energy needed to increase the temperature of 1 g of water by 1 Celsius varies depending on the starting temperature, and is in any case difficult to measure precisely. Accordingly there have been several definitions of the calorie:

15 °C calorie (cal15): the amount of energy required to warm 1 g of air-free water from 14.5 °C to 15.5 °C at a constant pressure of 101.325 kPa (1 atm). Experimental values of this calorie ranged from 4.1852 J to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.

20 °C calorie: the amount of energy required to warm 1 g of air-free water from 19.5 °C to 20.5 °C at a constant pressure of 101.325 kPa (1 atm). This is about 4.182 J.

4 °C calorie: the amount of energy required to warm 1 g of air-free water from 3.5 °C to 4.5 °C at a constant pressure of 101.325 kPa (1 atm).

Mean calorie: 1/100 of the amount of energy required to warm 1 g of air-free water from 0 °C to 100 °C at a constant pressure of 101.325 kPa (1 atm). This is about 4.190 J

The two perhaps most popular definitions used in older literature are the "15 °C calorie" and the "thermochemical calorie". Since the many different definitions are a source of confusion and error, all calories are now deprecated in favour of the SI unit for heat and energy: the joule (J).

Human fat tissue contains about 87% lipids, so that 1 kg of body-fat tissue has roughly the caloric energy of 870 g of pure fat, or 7800 kcal. In principle one has to create a 7800 kcal deficit or surplus between energy intake and use to lose or gain 1 kg of body-fat, respectively, or 3500 kcal per pound. [1] However, if one eats 7800 kcal more than the body needs, one won't necessarily gain 1 kg of fat, since muscle and other tissues may be built. In the same way, if one eats 7800 kcal less than their maintenance level, they may not lose 1 kg of fat, since muscle and sugars may be metabolized to generate energy.

USDA National Nutrient Database for Standard Reference - Release 19 Official, publicly available reference database and online search site. Includes 7,293 foods and is free to download and use. This database is the one used by most websites that provide calorie information, and forms the basis of the Canadian national nutrient database and others